This invention relates, in general, to a fuel cell system and, more particularly, to the detection of leakage current in the coolant of a fuel cell system.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
The electrically conductive elements sandwiching the MEAs may contain an array of grooves in the faces thereof for distributing the fuel cell""s gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. In the fuel cell stack, a plurality of cells are stacked together in electrical series while being separated one from the next by a gas impermeable, electrically conductive bipolar plate. Heretofore, the bipolar plate has served several functions including (1) as an electrically conductive gas separator element between two adjacent cells; (2) to distribute reactant gases across substantially the entire surface of the membrane; (3) to conduct electrical current between the anode of one cell and the cathode of the next adjacent cell in the stack; (4) to keep the reactant gases separated in order to prevent auto ignition; (5) to provide a support for the proton exchange membrane; and (6) in most cases, to provide internal cooling passages therein defined by internal heat exchange faces and through which a coolant flows to remove heat from the stack.
Current fuel cell technology requires a low conductivity (high resistance) coolant to prevent leakage current from flowing between the stack in the remainder of the system. Leakage current flowing through the coolant can cause short circuiting, induce galvanic corrosion and electrolyze the coolant, reducing engine efficiency. Generally non-corrosive coolants such as water, antifreeze, or mixtures thereof, etc., are used in the bipolar plates. Over time, however, the internal heat exchange faces of the bipolar plates begin to dissolve. The dissolution of even small parts of material from the bipolar plates into the coolant can cause the coolant to become excessively conductive, resulting in excessive leakage current.
Heretofore, coolant conductivity has been monitored using a sensor that is specific to conductivity measurement. The sensor indicates the level of conductivity (or resistivity) of the coolant whereupon an electronic controller decides whether the measured level is sufficient to prevent large leakage current. Therefore, the prior method does not measure the leakage current, only one potential cause of it. The sensors do not detect other faults, such as a short circuit across the stack. Also, because the coolant conductivity sensors are purchased and calibrated specifically for the coolant being used and require specific mounting hardware and orientation, they are relatively expensive and difficult to install. Finally, these sensors require a finite reaction time to make their measurements and have a potential to xe2x80x9cdriftxe2x80x9d in their measurement of conductivity over time, decreasing their ability to reliably detect conductivity under all circumstances.
The present invention provides a method and apparatus for detecting the presence of leakage current in the coolant of a fuel cell stack without using a coolant conductivity sensor.
The method of the present invention detects the presence of leakage current by disposing a fixed resistance from the negative terminal of the fuel cell stack to the fuel cell stack chassis and measuring a voltage between the positive and chassis which is electrically connected via the coolant path. Then, comparing the measured voltage to a first predetermined voltage limit, and reporting when the voltage is at or below the first predetermined voltage limit. Lower voltages indicate higher leakage currents in the coolant. In one aspect of the invention, the first predetermined voltage limit is zero volts, indicating a short circuit across the stack.
In another aspect method of the present invention, the coolant voltage is compared to a second predetermined voltage limit and a report is made when the coolant voltage is at or below that limit.
The apparatus of the present invention comprises a first voltage measuring device for measuring the coolant voltage between the positive and chassis which is electrically connected via the coolant path, and a fixed resistance disposed between the negative terminal and the stack chassis. In another aspect of apparatus of the present invention, a second voltage measuring device measures the fuel cell stack voltage between the positive terminal and the negative terminal of a fuel cell stack. The apparatus also comprises means for comparing the coolant voltage to a first predetermined voltage limit and, in one aspect, to a second predetermined voltage limit, and means for reporting when the coolant voltage is at or below the predetermined voltage limit(s).
According to an optional aspect of the present invention, the resistivity of the coolant is calculated by measuring the stack voltage from the positive terminal of the stack to the negative terminal of the stack, calculating the resistance of the coolant, and using the resistance of the coolant and the physical parameters of the system to calculate the resistivity of the coolant from the resistance. Once the resistivity is calculated, the conductivity can also be calculated by taking the reciprocal of the resistivity.
The present invention replaces the conductivity sensor with relatively cheap components, namely a resistor and one or more voltage measuring devices. Expense is further reduced because the invention can be adapted for installation on any system quickly and easily because it requires no specific orientation or calibration for the type of coolant being used. The detection of excessive leakage currents is more reliable because reaction time is faster than with the prior conductivity sensor and, further, devices that measure voltage are less likely to xe2x80x9cdriftxe2x80x9d over time. The present invention is also intended to detect other problems with the stack. For example, if any point in the system comes in contact with the stack (i.e., dropped hardware, rags, broken equipment), the system will detect this as a ground fault. The prior conductivity sensor indicates only that the coolant may be conductive.